Electronic part mounting apparatus used, for example, in the process of producing electronic equipments such as computers or mobile telephones are adapted, for example, such that an electronic part suction head is disposed vertically movably above an X-Y table on which a substrate is placed and electronic parts are mounted at predetermined positions on the substrate by sucking electronic parts such as semiconductor devices by the head. Accordingly, for mounting electronic parts exactly to predetermined positions on the substrate in such electronic part mounting apparatus, it is necessary to improve the positioning accuracy for the X-Y table, as well as improve the positioning accuracy of a head lifting mechanism for reciprocating the electronic part suction head in the vertical direction and, for this purpose, it is necessary to improve the positioning accuracy of a linear motion guiding device used as a linear guide for the head lifting mechanism.
Particularly, along with size-reduction of the electronic equipments themselves in recent years, size reduction of electronic parts mounted on a substrate and high integration degree of the substrate have been proceeded and, since the positioning accuracy upon mounting the electronic parts reaches the order of several μm, the positioning accuracy required for the linear motion guiding device has been increased more and more.
Further, for improving the production efficiency, the mounting speed has also tended to be increased and, for enabling mounting of electronic parts at such a speed, for example, of 0.5 to 0.1 sec or less for one cycle, a head lifting mechanism capable of vertically moving the electronic part suction head at a high speed is required and, for this purpose, the linear motion guiding device assembled into the head lifting mechanism also has to cope with high speed reciprocation of the head. Further, they are required not only for the linear motion guiding device used in the head lifting mechanism described above but also for the linear motion guiding device, for example, used as a bonding head lifting mechanism of a wire bonding apparatus.
By the way, for vertically moving, for example, an electronic part suction head of an electronic part mounting apparatus or a bonding head of a wire bonding apparatus accurately and at a high speed, it is necessary to increase the rigidity of a guide rail of a linear motion guiding device to decrease distortion or vibration caused to the guide rail. As a linear motion guiding device intended to increase the rigidity of such guide rail, a device disclosed, for example, in Japanese Published Unexamined Patent Application No. 175691/1987 (Japanese Published Examined Patent Application No. 44051/1994) has been known. Further, Japanese Published Unexamined Patent Application No. 62958/1999 discloses a technique of using cemented carbides as the rail materials for the guide rail.
However, in the linear motion guiding device disclosed in Japanese Published Unexamined Patent Application No. 175691/1987, the guide rail is formed of a ceramic material having a specific rigidity of 0.8×108 mm or more and its application to the electronic part mounting apparatus or the wire bonding apparatus has been difficult by the following reasons.
That is, most of the electronic part mounting apparatus and the wire bonding apparatus generally have a constitution in which a slider of the linear motion guiding device is fixed on a support bed or the like, while the guide rail is moved to reciprocate the head. On the contrary, since the linear motion guiding device disclosed in the publication above has a constitution of fixing both ends of the guide rail on the supporting bed or the like and moving the slider in use, it is not suitable to the application to the electronic part mounting apparatus or wire bonding apparatus described above.
Further, since most of the electronic part mounting apparatus conduct a series of steps from suction to mounting of electronic parts continuously, a so-called machine gun system of disposing plural guiding rails on a rotating drum to continuously mount electronic parts has been adopted. Accordingly, since vertical movement for mounting the electronic parts and, in addition, rotational acceleration due to the rotation of a drum synchronized therewith exert on the guide rails, inertia force generated by the own weight of the rails or head weight exerts as a bending moment on the guide rails. Particularly, when the cycle time for the vertical movement of the guide rail exceeds 0.2 sec, acceleration applied on the guide rail increases to as high as about several to ten G and, in addition, acceleration in the circumferential direction of the drum also reaches about several G. Accordingly, a sufficient strength is required for the guide rail used under such conditions regarding the composite acceleration described above and the inertia force generated by the own weight of the rail and the mass of the head.
However, in the guide rail formed of ceramics disclosed in the publication described above, while hardness and rigidity are high, the bending strength is not so high, and the bending strength is lower compared with guide rails formed of iron and steel materials such as bearing steels or stainless steels. Further, when a large bending moment is loaded on the guide rail, the guide rail is fractured in a case where the bending strength is insufficient even if the constituent material therefor is alumina ceramics, silicon carbide ceramics or silicon nitride. Accordingly, it is difficult to increase the speed of the apparatus by merely forming the guide rail of ceramics regarding the reliability in view of the strength(particularly, reliability to bending strength).
Further, the guide rail formed of a brittle material such as ceramics is also sensitive to the change of strength (stress concentration) depending on the rail shape and, in a case where attaching holes for attaching parts such as a head or recesses or the like for avoiding interference with attaching parts are provided to the guide rail, stresses tend to be concentrated to the portions. Accordingly, it is difficult to increase the speed of the apparatus by merely constituting the guide rails with ceramics in view of the reliability for the strength (particularly, bending strength).
Further, while the rigidity of the rail per se is increased by forming the rail material of ceramic material, surface contact pressure with a rolling member incorporated in a slider increases, which increases the load on the rolling member compared with a case of using a rail formed of steels. For example, when silicon nitride is used as the rail material and the rolling element is constituted with martensitic stainless steel, a difference of twice or more is caused for the hardness between them and wear of the rolling element is sometimes accelerated compared with a case of using a rail formed of steels.
On the other hand, as disclosed in Japanese Published Unexamined Patent Application No. 62958/1999, a material of high rigidity, that is, a material of high young's modulus as the material property includes cermet or cemented carbide. Cermet or cemented carbide has young's modulus as high as about 300 GPa-650 GPa compared with metal a material such as bearing steel(250 GPa) and it is high also compared with various ceramics(silicon nitride at about 250 GPa-350 GPa, alumina at about 350 GPa-420 GPa and silicon carbide at about 400 GPa-420 GPa). Accordingly, when the guide rail is formed of a cermet or cemented carbide having high young's modulus, rigidity of the guide rail can be increased. However, when the guide rail rotates while moving vertically at a high speed as in the electronic part mounting apparatus or wire bonding apparatus, large inertia force is generated by acceleration or own weight of the rail and the head weight, and the driving performance(cycle speed, response performance) is deteriorated by the inertia force. Further, since the inertia force is also increased when the density(mass) of the guide rail is large in this case, the bending strength of the guide rail is insufficient to sometimes result in fracture even in a case of using the cermet or cemented carbide as the rail material.
Further, when the ceramic material(particularly, usual silicon nitride) is used as the rail material, the heat conductivity is low and heat tends to be accumulated in the inside of the apparatus. That is, when the guide rail is formed of a ceramic material of poor heat conductivity such as silicon nitride, since the temperature on the sliding surface of the guide rail becomes higher during operation and the grease viscosity lowers due to the temperature elevation on the rail sliding surface compared with a case of using iron and steel material such as bearing steels as the rail material, formation of oil membranes between the rolling element and the surface of the rail groove is inhibited to cause wear or minute seizure of the rolling element. They cause generation of vibrations during operation of the linear motion guiding device, giving undesired effects on the accuracy in the repetitive positioning. Further, temperature elevation of the rail material accelerates thermal expansion of the guide rail, which also gives undesired effects on the accuracy in the repetitive positioning.
Further, for obtaining stable accuracy in the repetitive positioning for a long time, a rail material of good heat dissipation is necessary. Particularly, the speed as the operation condition increases more and more and, in addition, demand for the accuracy in the repetitive positioning becomes more stringent in the linear motion guiding device. For satisfying such requirements, it necessitates a linear motion guiding device capable of ensuring the positioning accuracy for a long time with less positional displacement caused by elastic deformation of the rail material, as well as with less occurrence of thermal expansion or wear of the rolling element by the improvement in the heat dissipation of the apparatus.
By the way, since various chemicals are used in the cleaning step or film deposition step upon manufacture of semiconductors, liquid crystal panels and hard discs, it is required for the rolling device used in such steps to operate with no troubles even in a corrosive atmosphere such as in an atmosphere of chemicals. Further, in view of increase in the diameter of wafers or liquid crystal panels, it results in the requirement for the rolling device to support larger load.
Japanese Published Unexamined Patent Application No. 121488/1996 discloses a corrosion resistant rolling bearing in which an outer ring is formed of ceramic material manufactured by an atmospheric pressure sintering process and an inner ring is formed of a ceramics material manufactured by a gas pressure sintering process or an HIP process.
Further, Japanese Published Unexamined Patent Application No. 82426/1998 discloses a rolling bearing formed of ceramics of excellent corrosion resistance in which each of an inner ring, an outer ring and rolling elements is constituted with silicon carbide.
On the other hand, in jet engines or gas turbines, since the efficiency has been improved in view of energy saving and environmental problem, it is required for the rolling device used therefor to operate with no troubles under higher load and at higher temperature.
However, in the rolling bearing described in Japanese Published Unexamined Patent Application No. 121488/1996, since the outer ring is manufactured by an atmospheric pressure sintering process, it involves the following problems. That is, a member manufactured by the atmospheric pressure sintering process has low strength and fracture toughness in which micro-cracks tend to develop starting from the surface or internal. Accordingly, a great amount of abrasion powder is formed or cracks are resulted to sometimes shorten the life of the rolling bearing.
Particularly, when the rolling bearing supports a radial load, since the load is concentrated to a load region of the outer ring, cracks propagate easily in the load region of the outer ring manufactured by the atmospheric pressure sintering process even under a slight load to sometimes shorten the life extremely.
Further, in a case where the inner ring, the outer ring and the rolling elements are constituted with silicon carbide as in the rolling bearing described in Japanese Published Unexamined Patent Application No. 82426/1998, although the corrosion resistance is excellent, it involves a problem that the strength and the fracture toughness are low. When a load exerts to some extent on the rolling bearing, cracks propagate on the surface or through the entire portion to sometimes cause flaking or cracking.
Particularly, when the rolling bearing supports a radial load, since the load is concentrated in the load region of the outer ring, flaking and cracking occur to sometimes shorten the life extremely even under a slight load.
Further, Japanese Published Examined Patent Application No. 30788/1995 proposes a rolling bearing having rolling elements between an inner ring fitted to a steel shaft and an outer ring held on a housing, in which the material for the inner ring is formed of a material of lower linear expansion coefficient than the material for the outer ring and the linear expansion coefficient of the material for the inner ring is less than the linear expansion coefficient for the material of a steel shaft fitted to the inner ring.
In machine tools or various kinds of spindles, rotation has tended to be increased more and more in recent years, and the rolling bearing for supporting the rotational portion, for example, of the machine tools is also required to operate at high accuracy and under severe working conditions. Further, also in a usual bearing support device, since heat of the outer ring along with heat generation tends to be dissipated relatively easily through a housing but the heat of the inner ring is less dissipated from the side of the shaft, the temperature of the inner ring tends to be higher compared with the outer ring.
However, in an existent rolling bearing in which the outer ring and the inner ring are formed of an identical material, for example, a high carbon chromium bearing steel material such as bearing steel (SUJ2), when the temperature of the inner ring is higher than that of the outer ring due to the heat generation of the bearing or the heat from the outside to cause a temperature difference between the outer ring and the inner ring of the bearing, an internal gap of the bearing is decreased compared with a case before heat generation. Accordingly, under severe working conditions at high speed rotation, particularly, the radial gap of the bearing is excessively small or a preload becomes excessive by the change of the gap to sometimes bring about seizure or shorten the working life extremely.
Usually, in a case where the rotational speed is constant, it may suffice that a rolling bearing which was previously compensated so as to give an optimal gap or an optimal preload under the specified working conditions is selected and assembled. However, in a case where rotational conditions changes variously, and heat generation is large in the inside of the bearing or external heat is conducted to cause a temperature difference in the inside of the bearing, the gap in the inside of the bearing or the preload caused by the change of the gap may be adjusted by an external force(for example, by oil pressure mechanism) by detecting the temperature of the bearing assembled into the rotational device, but this involves a drawback that the device is complicated and becomes expensive.
Further, in the technique described in Japanese Published Examined Patent Application No. 30788/1995, since the material for the inner ring is formed of a material of smaller linear expansion coefficient than that of the material for the outer ring, and since the thermal expansion coefficient of the inner ring is smaller than the thermal expansion coefficient for the material of the steel shaft fitted to the inner ring, change of the gap is smaller compared with the case where the inner ring and the outer ring are formed of an identical material.
However, as the rotational speed increases and the heat generation increases to increase the temperature gradient in the rolling device, since the rolling element is formed of a bearing steel of the same material as that for the outer ring and the amount of thermal expansion is large, even if the amount of thermal expansion of the inner ring is less than the amount of the thermal expansion of the outer ring, the gap becomes insufficient to cause seizure or to sometimes shorten the life excessively.
Further, while Japanese Published Unexamined Patent Application No. 205276/2000 discloses a rolling bearing in which the heat conductivity of the ceramic material constituting the outer ring is made larger than the heat conductivity of the ceramic material constituting the inner ring and the rolling element, the rolling bearing described in this publication involves the following problems. That is, since some of the ceramic materials are insufficient in the thermal impact resistance or bending strength, when they are used in a high temperature atmosphere or high temperature/corrosive atmosphere, a temperature gradient is caused in the bearing and thermal stresses are generated by the temperature gradient. Then, micro-cracks propagate on the surfaces of the outer ring or the inner ring to sometimes form a great amount of abrasion powder, or cracks penetrate the member to cause breakage to shorten the working life of the rolling bearing.
On the other hand, since a rolling bearing used in a molten metal plating apparatus is used in a state immersed in a molten metal, it is required to be excellent in corrosion resistance to the molten metal. The rolling bearing described above is generally constituted with an iron and steel material. However, since the corrosive property of the molten metal to the iron and steel material is extremely strong and the level of the corrosion resistance of the iron and steel material directly gives an effect on the rolling life of the rolling bearing, a rolling bearing in which a portion in contact with the molten metal is constituted with a ceramic material has been proposed(for example, in Japanese Published Unexamined Utility Model Application No. 89428/1988 and Japanese Published Unexamined Utility Model Application No. 90852/1986).
However, although Japanese Published Unexamined Utility Model Application No. 89428/1988 and Japanese Published Unexamined Utility Model Application No, 90852/1986 disclose the names of various ceramics materials constituting the rolling bearing(Si3N4, SiC, Al2O3 and sialon), the thermal impact resistance value and the bending strength thereof are not described at all. Even when the rolling bearing is constituted with Si3N4, SiC, Al2O3 or sialon, when the thermal impact resistance value or bending strength is insufficient, micro-cracks propagates on the surface of the constituent members to cause a great amount of abrasion powder or cracks penetrate the constituent members to sometimes cause breakage.